1. Field of the Invention
The invention relates to a brake pressure booster and master cylinder assembly, comprising a booster casing in which a movable wall separates two compartment from each other, a control valve which connects the two compartments with each other when it is in an inoperative position and separates them when in an operative position, connecting one of them to a source of pressure which differs from the pressure in the other compartment, a cylinder casing attached to the booster casing, projecting into the same, and containing at least one pressure piston which defines a pressure chamber for pressurizing a brake circuit, an input piston which has a larger effective surface than the pressure piston, defines an input chamber, and is movable together with the movable wall, and a relief valve adapted to relieve the input chamber of pressure in case of failure of the brake pressure booster.
2. Description of the Prior Art
In a known assembly of this kind (DE-A-2 460 529) the cylinder casing is flanged to the booster casing by a flange formed at one end of the cylinder casing, and it comprises a short end portion which is centered in the booster casing. Primary and secondary pressure chambers are formed axially behind each other in the cylinder casing and each defined by a pressure piston. A brake circuit of a motor vehicle is adapted to be connected to each pressure chamber. An input chamber is formed between the primary pressure chamber and the end of the cylinder casing fastened to the booster casing, the diameter of the input chamber is greater than that of the primary pressure chamber. The input chamber contains an input piston connected to the movable wall in the booster casing by a push rod. The input chamber communicates with a connection for a pressure fluid reservoir through a relief valve which normally is kept closed by vacuum prevailing in the booster casing when the booster is ready for operation. Upon actuation of the brake, the pressure rod moves the input piston which thus displaces brake fluid in the input chamber, the brake fluid in turn advancing the pressure piston at the primary end. The distance by which this pressure piston is moved forward is greater than the distance travelled by the input piston at the ratio of the effective surfaces of the input piston and the pressure piston. Therefore, there is a stroke ratio between the input piston and the pressure piston at the primary end when the brake pressure booster is intact and the relief valve consequently is closed. The pressure piston at the secondary end has an effective surface of the same size as the pressure piston at the primary end and, therefore, is moved by the same distance as this piston. If the brake pressure booster should fail, the relief valve opens so that actuation of the brake will cause brake fluid displaced by the input piston to flow off into the pressure fluid reservoir without any increase in pressure taking place in the input chamber. The input piston is formed with an axial extension by which it directly pushes the pressure piston at the primary end so that this piston will be moved only by the same distance as the input piston. In case of failure of the booster, therefore, the actuating force required for the joint displacement of the input piston and the pressure piston at the primary end no longer is determined by the effective surface of the input piston but instead by the smaller surface of the pressure piston at the primary end.
Another known master cylinder assembly (US-A-4 086 770) comprises a cylinder casing which is likewise suited for attachment to a brake pressure booster and comprises two pressure pistons in tandem arrangement and an input piston connected upstream of the pressure piston at the primary end. The input piston again has a larger effective surface than the pressure piston at the primary end. However it is formed in one piece with the same so that both of them can only move together. Upon actuation of the brake, the input piston displaces a larger volume than released by the pressure piston. Consequently brake fluid flows from the input chamber past a lip seal of the pressure piston at the primary end into the associated pressure chamber whereby the brake circuit connected to the same is filled rapidly. When the pressure in the pressure chamber at the primary end exceeds a certain value, a relief valve opens to establish communication between the input chamber and a pressure fluid reservoir whereby the input chamber is relieved of pressure and the further power needed to actuate the brake is determined only by the effective surface of the pressure piston at the primary end.
Both known assemblies described above are of great structural length, as measured from the rear end of the booster casing to the front end of the cylinder casing. Mass forces of inertia resulting from vibrations and acting on the cylinder casing and the structural members it houses as well as on the pressure fluid reservoir attached to the same may cause great bending moments in the area of the connection between the cylinder casing and the booster casing so that the entire booster casing must be given a sturdy structure as it usually presents the only connection between the cylinder casing and the supporting portion of the associated vehicle.